1. Field of the Invention
This invention relates to a fixing apparatus which is provided in an image forming apparatus such as a copying machine or a printer, for fixing a developer-image on a paper sheet.
2. Description of the Related Art
In an image forming apparatus utilizing a digital technique, e.g., an electronic copying machine, when a document table on which an original is placed is exposed, light reflected from the document table is guided to an optical/electrical converting element such as a CCD (charge coupled device).
The CCD outputs an image signal corresponding to an image of the original. A laser beam corresponding to the image signal is radiated onto a photosensitive drum, thereby forming an electrostatic latent image on a circumferential peripheral surface of the photosensitive drum. The electrostatic latent image is visualized by developer. To the photosensitive drum, a paper sheet is fed in accordance with rotation of the photosensitive drum. The above visualized image (developer-image) on the photosensitive drum is transferred onto the paper sheet. Then, the paper sheet is fed to a fixing apparatus.
The fixing apparatus comprises a heat roller and a pressure roller which rotates along with the heat roller while being in pressure-contact with the heat roller. The fixing apparatus holds the paper sheet between the heat and pressure rollers, and fixes the developer onto the paper sheet due to heat of the heat roller, while transferring the paper sheet.
In order for the heat roller to generate heat, there is a method of carrying out induction heating. In the induction heating, a high-frequency current is made to flow through a coil, as a result of which a high-frequency magnetic field is generated from the coil, and an eddy current is generated by the heat roller due to the high-frequency magnetic field. Consequently, the heat roller generates heat by itself due to Joule heat generated by the eddy current.
In a fixing apparatus utilizes such induction heating, a first coil is provided in a position corresponding to a substantially center portion of the heat roller in the axial direction thereof, and second coils are provided in positions corresponding to the both end portions of the heat roller in the axial direction thereof, and the first and second coils are selectively driven.
To be more specific, when a paper sheet having a small size (A4R) is applied, it contacts only the substantially center portion of the heat roller in the axial direction thereof. In this case, only the first coil is driven, since it suffices that only the substantially center portion of the heat roller is heated (since the both end portions are not required to be heated).
When a paper sheet having a regular size (A4) is applied, it contacts the entire area of the heat roller in the axial direction. In this case, both the first and second coils are driven in order that the entire area of the heat roller be heated. However, actually, the first and second coils are alternately driven so that the power consumption falls within a rated power.
When the first and second coils are alternately driven, it is necessary to uniformly maintain the heat of the entire area of the heat roller.
However, in the case where the amount of heat transmitted to the paper sheet is large, or in the case where a heat roller having a small heat capacity, e.g. a heat roller having a thin structure, is adopted in order to shorten the time period required for warming up the fixing apparatus, the temperature of the substantially center portion of the heat roller lowers when driving of the first coil is stopped, and the temperatures of the both end portions of the heat roller lower when driving of the second coil is stopped. Such lowering of the temperature of the substantially center portion or the end portions adversely affects the fixing operation of the fixing apparatus.